Submitted to: Plant Disease
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/21/2021
Publication Date: 3/29/2021
Citation: Davis, E.A., Weiland, G.E., Scagel, C.F. 2021. Optimizing inoculum production methods for infesting soil with phytophthora species. Plant Disease. https://doi.org/10.1094/PDIS-12-20-2698-RE.
Interpretive Summary: Phytophthora inoculum is used to cause Phytophthora root rot disease of rhododendron shrubs in plant pathology experiments. However, it often takes more than 6 weeks to produce inoculum from our standard method and the amount of inoculum varies significantly from batch to batch. A more reliable method is needed to produce consistent inoculum more quickly for future studies. We quantified how much inoculum varied from batch to batch in our standard method and determined that an air drying step reduced inoculum viability when inoculum moisture content was low. Based on these results, we developed a new inoculum production method that can be completed more quickly and is more consistent among batches than our standard method. These results are important for other plant pathologists because they help explain why Phytophthora inoculum varies from batch to batch and provide a new method for producing inoculum.
Technical Abstract: Inoculum production is an important part of conducting research with soilborne Phytophthora species. One common method is to incubate Phytophthora cultures in nutrient-amended vermiculite. However, inoculum levels often vary among batches of inoculum even when production methods remain the same and incubation typically takes = 6 weeks, increasing risks for delayed experiments if the resulting inoculum level is too low. A more reliable and rapid method is needed for future studies. Experiments were conducted to: (1) determine inoculum levels of P. cinnamomi and P. plurivora after incubation in V8 juice-amended vermiculite (standard method); (2) evaluate how inoculum viability was affected by air drying; (3) develop a modified method that takes less time to produce a vermiculite-based inoculum; and (4) evaluate the effect of storage on inoculum viability. Results showed that the standard method produced inoculum levels from 716 to 1808 colony forming units/g and that drying to < 78% moisture content significantly reduced viability. The modified method used 2-week-old Phytophthora cultures to infest vermiculite at 80% moisture content and produced inoculum levels from 214 to 525 CFU/g. Although inoculum levels from the modified method were lower than the standard method, inoculum levels were more consistent between trials and it was 6 to 8 weeks faster. Production with the modified method can also be easily scaled up by infesting a greater volume of vermiculite with additional cultures of Phytophthora. These results are important because they help explain variability in soilborne Phytophthora inoculum production and storage, and provide a new method for producing inoculum more quickly.